tangent bundle
Let be a differentiable manifold. Let the tangent bundle of be(as a set) the disjoint union of all the tangent spaces
to , i.e., the set of pairs
This naturally has a manifold structure, given as follows. For , is obviously isomorphic
to , and is thus obviously a manifold. By the definition of a differentiable manifold, for any , there is a neighborhood of and a diffeomorphism . Since this map is a diffeomorphism, its derivative
is an isomorphism
at all points. Thus is bijective
, which endows with a natural structure of a differentiable manifold. Since the transition maps for are differentiable
, they are for as well, and is a differentiable manifold. In fact, the projection forgetting the tangent vector and remembering the point, is a vector bundle. A vector field on is simply a section
of this bundle.
The tangent bundle is functorial in the obvious sense: If is differentiable, we get a map , defined by on the base, and its derivative on the fibers.